Under high speed conditions, the outer diameter of the tire increases due to centrifugal forces generated by steel cord belt package and tread. Such a diameter increase or tire growth increases the pantographic movements of the belt edge cords leading to the crack initiations, crack propagations and at the end belt edge separations.
The cap ply layer wound on belt package circumferentially prevents excessive tire growth under high speed conditions by applying compressive forces (restraining force) on heavy belt package made of cross ply steel cord layers.
Currently, most widely used cap ply materials are multi-ply nylon 6.6 layers or single layer hybrid cords which are spirally wound on belt package at 0 to 5 degrees to equatorial plane of the tire. In addition to nylon 6.6 and aramid/nylon 6.6 hybrid cords, there are also some polyester (polyethylene terephthalate, PET) applications as cap ply. But due to high initial modulus of PET, there is a risk of tight cord formation after lifting which means contact with steel cords of the top belt layer leading to cord breaks under dynamic conditions.
Nylon cords have excellent fatigue resistance under bending and axial compression, and bielastic tensile characteristic enabling easy processing during tire building. Additionally, shrinkforce generation with increasing service temperature under high speed conditions improves belt edge separation resistance and high speed durability. But low glass transition temperature of nylon causes flatspot problems in tire during parking after high speed driving. Other potential drawback of multiply nylon cap ply assembly is its high rubber gauge which cause to increase the rolling resistance of the tire by hysteresis (heat build up).
As mentioned above, the hybrid cords comprising high and low modulus yarns having bi-elastic tensile behaviour are also used as cap ply in high speed tires. The low modulus component of hybrid cord enables easy belt package lifting without tight cord formation due to its high extensibility and the high modulus component becomes effective in service conditions. By using hybrid cords as cap ply, the total thickness of the cap ply layer and rubber content can be reduced, and the high modulus component of the hybrid cord enhances the restraining force and improves the high speed durability. But existence of nylon causes also some flatspot. As cap ply, hybrid cord containing aramid is a expensive solution.
On the other hand, cap ply cords made of ultra high modulus yarns having high twist have been also used as tire reinforcement to eliminate flatspot and improve high speed performance, but high level of cord twist results in drastic modulus and breaking strength(tenacity) losses.
U.S. Pat. No. 6,799,618 describes a textile cap ply structure which is superimposed radially outwardly to the belt assembly is reinforced with cords being made of materials including nylon and aramid. At initial elongation of the cords, the primary load bearing yarns are nylon yarns, and after appreciable elongation the primary load bearing yarns are the aramid yarns.
U.S. Pat. No. 7,584,774 describes a belt reinforcing layer (cap ply) dieposed outside the belt in the radial direction, characterized in that the belt reinforcing layer is formed by continuously and spirally winding a polyethylene terephthalate(PET) cord in a circumferential direction of the tire, and this cord has an elastic modulus of not less than 2.5 mN/dtex. % under a load of 29.4N measured at 160° C. Due to high initial modulus of the PET cord, lifting must be less tha 2% in this applications to prevent excessive tight cord formations.
U.S. Pat. No. 2013/0025758 describes a high performace pneumatic radial tire for passenger cars which uses a hybrid cord composed of two ply aramid yarn and single ply nylon yarn having different twist counts for the first twist and different twist counts for the second twist, as a cap ply, and thereby has improved high speed durability and steering stability.